"Eyeball Earth" could melt to form a lobster imprint.

The most common stars in our galaxy are dwarfs—smaller, reddish stars that emit far less light than our Sun. Because of this reduced output, the habitable zones of these stars are quite close-in. This means that any planets in the habitable zone will be close enough to become tidally locked, only showing a single face to the star, just as the Moon keeps just one side pointed at the Earth.

This led to the proposal that any watery worlds in the vicinity could form what's called an "Eyeball Earth." Directly under the local star, the light would be intense enough to melt a circular patch of water, while the rest of the planet would remain locked in a deep freeze.

Exoplanet climates go 3D

Now, a full model of the ocean and climate of a tidally locked planet suggests that the ice and oceans of these planets would be dynamic, distorting the dark pupil of the eyeball into something resembling a lobster. There's also good news and bad news for life on these watery planets. Although the analysis suggests a narrower habitable zone, more of the planet's surface would have the potential to support life.

The new paper, written by two researchers at Peking University, represents the next step in our analysis of exoplanets. Initially, estimates of habitability—defined as temperatures that could allow liquid water on the planet surface—were based on a single slice through the atmosphere, with things like scattering and greenhouse gasses setting the amount of light that reaches the planet's surface. But, in the real world, atmospheres form clouds, distribute heat through winds and convection, and exhibit other sorts of complex behavior.

Further Reading

These are the sorts of things that are handled in the full, three-dimensional climate models built to study the Earth, so effort was put into adapting these models to handle exoplanets that differed significantly from Earth. It was the result of one of these models that first predicted the existence of Eyeball Earths.

But these models didn't capture a critical part of the distribution of heat on the Earth: the ocean circulation. Instead, it treated the entire ocean as a two-dimensional slab. The new study corrects for that by using a coupled ocean-atmosphere climate model, the Community Climate System Model version 3.

For their planet, they used Gliese 581g, a potentially Earth-like planet orbiting in the habitable zone of an M dwarf star 20 light years away. The exoplanet is only about 1.5 times the size of Earth, and its orbit takes about 36 days. Critically for the model, it's close enough to its host star to receive 866 Watts/square meter at the top of its atmosphere (the Earth receives 1,366). Since we don't know what Gliese 581g's atmosphere looks like, the authors assumed an Earth-like composition, but varied the amount of CO2 to change the intensity of the greenhouse effect. The planet was assumed to be covered in a deep ocean.

A melted lobster

After giving the model 1,100 years to come to equilibrium, the authors sampled a century of its climate. With carbon dioxide concentrations similar to the Earth's (330 parts per million in the model), the eyeball vanished. That's because ocean currents formed along the equator and brought in ice from the west that split the eyeball into two lobes that flanked the equator—the claws of the lobster. The currents then transferred heat to the east, which melted the ice to form the lobster's tail. Critically, the coldest spot on the planet ended up being -60°C—too warm for carbon dioxide to freeze out as dry ice, which would kill the greenhouse warming and turn the planet into a snowball.

A smaller but otherwise identical pattern formed when the CO2 concentrations were dropped to 3ppm. Raise them to 200,000ppm, and the entire planet melted.

In addition to the ocean current that altered ice distribution, a thermohaline circulation (like the ocean conveyor belt on Earth) formed, which sent warmer water toward the poles. However, most of the action took place above 400m from the ocean's surface; below that, temperatures were pretty homogeneous. In the atmosphere, a jet stream also formed over the equator, which also distributed some heat to the unlit side of the planet.

To test how habitability changed with distance from the star, the researchers fixed the amount of CO2 to 330ppm, but varied the amount of energy reaching the top of the atmosphere. They found that as the amount of incoming light dropped, the outer edge of the habitable zone—where all the water froze and a snowball planet formed—was actually closer in than a model without ocean circulation put it. On the inner side, the planet also melted at the point where a planet with a simplified ocean still had 70 percent of its surface covered in ice. This rapid melting increases the water vapor in the air, making it easier for the planet to enter a runaway greenhouse state.

So the new model suggests the habitable zone of watery planets near M dwarfs is a bit more narrow than previous studies had suggested. That's the bad news for life. The good news is that, in this model, the ice never got very thick on the dayside of the planet. At 330ppm, most of the day side only had ice that was 3m thick. That's thin enough to allow light to reach the water underneath, meaning photosynthesis is a possibility over the entire dayside of the planet. That's in sharp contrast to earlier models.

Although this model is a major improvement, it still lacks a key feature that's likely to exist on planets: continents, or at least features on the seafloor that differ greatly in height. These will radically alter the currents on the planet, and thus radically alter the distribution of heat within the ocean. Unfortunately, we're even less likely to know anything about continents on exoplanets than we are about the composition of their atmospheres. Still, a few model runs with different configurations might provide some interesting information.

There is a serious translation error in this article, perhaps carried over from other English language sources. The researches didn't describe the warm area has having a lobster shape, but rather a "美味的烤肉串蝎子" :

The weather patterns would be a lot more stable, given they emerge from the varying heat of the sun constantly moving around the earth. Would it even rain? How would vegetation renew without clear seasonal growing cycles?

I've been thinking, though. This contracted 'Eyeball Earth' model reminds me more of what I'd call a 'Couchpotato Earth', instead (y'know, laying on its side?)

If the simulation that has been ran is accurate*, and the uber-cold and uber-hot zones are smaller than most of the hemisphere of a world in question, would it be fair to argue that it's a case of a non-polar ice cap paired with a non-polar desert?

*: I know, I know, A) it's just a simulation, and B) as the article states, more Earth-like terrain conditions like continents, mountains, etc. haven't been ran against the simulation. This probably proves very little so far, I'm bringing this up for the joy of academic discussion.

EDIT: That brings me to another question: how would mountains work on a tidally-locked world like this? Would their permashadow have some interesting biosphere implications? Now I'm all curious...

The weather patterns would be a lot more stable, given they emerge from the varying heat of the sun constantly moving around the earth. Would it even rain? How would vegetation renew without clear seasonal growing cycles?

No reason why it wouldn't rain. I would suspect that, barring underwater features that shift currents around, you'd have constant rain in some areas. Without varying energy input from the sun and without the coriolis, the weather patterns should be a lot more linear. Although, depending on axial tilt and how elliptical the orbit is, there could very well be varying energy input from the sun -- in this case, monthly cycles instead of annual (since it's years are only 36 days long)

As for vegetation renewing without clear seasonal growing cycles... they wouldn't evolve to work with seasonal cycles.

Edit: durr, tidally locked. I'm pretty sure that means no axial tilt as we know it

EDIT: That brings me to another question: how would mountains work on a tidally-locked world like this? Would their permashadow have some interesting biosphere implications? Now I'm all curious...

Depending on how long it takes to run these simulations, they'll probably just "paint" features onto the planet in a few specific patterns (one big sea-mount, east-west stripes, etc.) then let it run a few times with random combinations. Geologists might have something to say about how nonmoving sunlight would affect tectonics (we can actually detect the slight contraction of the Earth's crust during an eclipse).

EDIT: That brings me to another question: how would mountains work on a tidally-locked world like this? Would their permashadow have some interesting biosphere implications? Now I'm all curious...

Depending on how long it takes to run these simulations, they'll probably just "paint" features onto the planet in a few specific patterns (one big sea-mount, east-west stripes, etc.) then let it run a few times with random combinations. Geologists might have something to say about how nonmoving sunlight would affect tectonics (we can actually detect the slight contraction of the Earth's crust during an eclipse).

I assume the main effects have nothing to do with sunlight and far more to do with gravitational differences.

Either way, it's really fascinating that climatological/geological simulations have progressed as far as they have.

After giving the model 1,100 years to come to equilibrium, the authors sampled a century of its climate.

Not to pick nits, but the Earth's environment has changed radically over its billions of years of history. Are there ways to tweak the model to figure out the evolution of a primordial atmosphere in a tidally-locked world without jumping right to the nitrox air that seems to bathe every planet in the Star Trek universe?

After giving the model 1,100 years to come to equilibrium, the authors sampled a century of its climate.

Not to pick nits, but the Earth's environment has changed radically over its billions of years of history. Are there ways to tweak the model to figure out the evolution of a primordial atmosphere in a tidally-locked world without jumping right to the nitrox air that seems to bathe every planet in the Star Trek universe?

If we could find spectrographic evidence of an exoplanet undergoing that very process I'm sure that would be the holy grail for researchers, especially if the trajectory hints that said primoridial atmosphere is moving toward the standard Sci-Fi nitrox mix (StarCraft has this on quite a few worlds too, as does the open-beta game Starbound.)

After giving the model 1,100 years to come to equilibrium, the authors sampled a century of its climate.

Not to pick nits, but the Earth's environment has changed radically over its billions of years of history. Are there ways to tweak the model to figure out the evolution of a primordial atmosphere in a tidally-locked world without jumping right to the nitrox air that seems to bathe every planet in the Star Trek universe?

If we could find spectrographic evidence of an exoplanet undergoing that very process I'm sure that would be the holy grail for researchers, especially if the trajectory hints that said primoridial atmosphere is moving toward the standard Sci-Fi nitrox mix (StarCraft has this on quite a few worlds too, as does the open-beta game Starbound.)

I've seen this convention just about everywhere except for the 4X game Pax Imperia which had four atmosphere types, and you could pick any one of them as your species' natural habitat. Too bad the idea seemed to die out with that one, old game.

After giving the model 1,100 years to come to equilibrium, the authors sampled a century of its climate.

Not to pick nits, but the Earth's environment has changed radically over its billions of years of history. Are there ways to tweak the model to figure out the evolution of a primordial atmosphere in a tidally-locked world without jumping right to the nitrox air that seems to bathe every planet in the Star Trek universe?

If we could find spectrographic evidence of an exoplanet undergoing that very process I'm sure that would be the holy grail for researchers, especially if the trajectory hints that said primoridial atmosphere is moving toward the standard Sci-Fi nitrox mix (StarCraft has this on quite a few worlds too, as does the open-beta game Starbound.)

Another game to add to my watch list!

On-topic: While this is a nice 'breakthrough', I am curious, is this going to get further study? More specifically, will the JWST, or similar Kepler-like satellites be launched to figure out the composition of these exoplanets?

After giving the model 1,100 years to come to equilibrium, the authors sampled a century of its climate.

Not to pick nits, but the Earth's environment has changed radically over its billions of years of history. Are there ways to tweak the model to figure out the evolution of a primordial atmosphere in a tidally-locked world without jumping right to the nitrox air that seems to bathe every planet in the Star Trek universe?

If we could find spectrographic evidence of an exoplanet undergoing that very process I'm sure that would be the holy grail for researchers, especially if the trajectory hints that said primoridial atmosphere is moving toward the standard Sci-Fi nitrox mix (StarCraft has this on quite a few worlds too, as does the open-beta game Starbound.)

Another game to add to my watch list!

On-topic: While this is a nice 'breakthrough', I am curious, is this going to get further study? More specifically, will the JWST, or similar Kepler-like satellites be launched to figure out the composition of these exoplanets?

Off-topic: Starbound is a particularly good one. A recent patch made it impossible to breathe on Moon or Asteroid Field biomes without the proper equipment (there's also the whole 'staying warm' thing, but Nanostoves tend to help with that a bit!) It can be bought on Steam for like $15 (there was a holiday sale, but I think it may be over. Oh, well.)

On-topic: Yeah, this is the sort of modelling that I would think would help us learn much more about our (relatively) close neighboring systems. I second this question.

The weather patterns would be a lot more stable, given they emerge from the varying heat of the sun constantly moving around the earth. Would it even rain? How would vegetation renew without clear seasonal growing cycles?

After giving the model 1,100 years to come to equilibrium, the authors sampled a century of its climate.

Not to pick nits, but the Earth's environment has changed radically over its billions of years of history. Are there ways to tweak the model to figure out the evolution of a primordial atmosphere in a tidally-locked world without jumping right to the nitrox air that seems to bathe every planet in the Star Trek universe?

If we could find spectrographic evidence of an exoplanet undergoing that very process I'm sure that would be the holy grail for researchers, especially if the trajectory hints that said primoridial atmosphere is moving toward the standard Sci-Fi nitrox mix (StarCraft has this on quite a few worlds too, as does the open-beta game Starbound.)

I've seen this convention just about everywhere except for the 4X game Pax Imperia which had four atmosphere types, and you could pick any one of them as your species' natural habitat. Too bad the idea seemed to die out with that one, old game.

IIRC the Space Empire series did something similar with a choice between your races habitat being terrestrial, gas giant, or iceball worlds. I'm not sure because it's been a long time since i played one of them though; too much micromanagement for my idea of fun.

On-topic: While this is a nice 'breakthrough', I am curious, is this going to get further study? More specifically, will the JWST, or similar Kepler-like satellites be launched to figure out the composition of these exoplanets?

There has been talk about telescopes designed specifically to do this sort of thing for more than a decade (since about the mid-90s, when the first exoplanets were discovered, actually, although the major thing then was finding terrestrial planets to begin with). Mostly they've foundered on high costs and James Webb (which as I understand it isn't able to do those sorts of measurements), although perhaps the next big telescope will be something like that. Supposedly, there's a generational divide with the older researchers mostly being into galactic and stellar astronomy and the younger people being more interested in exoplanets, although who knows how true that is.

On another note, I actually saw one of these guys give a presentation on this very topic a couple of months ago at school. I recall there being some rather interesting nonlinear effects as you increased CO2 concentration, not sure if those made it into the final paper. (I'd check, but the DOI appears to have an error, or at least when I click the link I get an error message).

To jump on the game discussion...Starbound has never, so far as I know, been on sale during the present Steam sale (it's always possible it could go up in the next couple of days), probably because it just came out. It is pretty fun, in the vein of Terrestria or (more distantly) Minecraft (or so I understand--I haven't played either of those two games). Varying planetary environments being "the best" has been a thing in multiple 4X-style games--I particularly recall Haegemonia, where the three main races all had different preferences, and Masters of Orion 3, which also had varying environmental preferences (for instance, the gas-giant dwelling species would find the deep-sea residing species planets unpalatable, and vice-versa). It's kind of a pity that MoO3 has this awful reputation, because there were actually some fairly interesting ideas in it.

Hasn't been on sale anywhere, yet, which is why I haven't picked up a copy. I did buy a copy for my son who loves Terreria and he loves Starbound just as much.A thumbnail description of it as "terreria in space" seems pretty apt. It feels a bit like JackJunk, though that's just from my passive observations.

(The similarity to Terreria is also why I haven't bought it for myself... never could get into it.)

Hasn't been on sale anywhere, yet, which is why I haven't picked up a copy. I did buy a copy for my son who loves Terreria and he loves Starbound just as much.A thumbnail description of it as "terreria in space" seems pretty apt. It feels a bit like JackJunk, though that's just from my passive observations.

(The similarity to Terreria is also why I haven't bought it for myself... never could get into it.)

False. Over Christmas it was $7.99 on Steam. Which, is why I was so dejected; I bought it a week before that, at $14.99. I'm not saying it's not absolutely worth it, even in its obvious beta state, but still, I'd rather pay less than more. Oh, well.

As far as Pax Imperium is concerned, I really wish they'd make a reboot of that one. It was too complicated for me as a younger child, but now if it's given the XCOM: Enemy Unknown treatment, it might be pretty awesome.

But, enough out of me on games. Personally, I think understanding what's out there is a lot more interesting than pixels on my own screen. I'd love to see the modified source to that environmental simulator, all the same, though...

EDIT: Dang it. I was thinking of something else. It's on sale on Steam for $14.99. Please, disregard me when it comes to prices on games, I cannot be trusted to dual-wield Mk.I Human Eyeballs.

Hasn't been on sale anywhere, yet, which is why I haven't picked up a copy. I did buy a copy for my son who loves Terreria and he loves Starbound just as much.A thumbnail description of it as "terreria in space" seems pretty apt. It feels a bit like JackJunk, though that's just from my passive observations.

(The similarity to Terreria is also why I haven't bought it for myself... never could get into it.)

Considering its an early access game I wouldn't expect it to go on sale anytime soon (and actually, would encourage to buy it at the current price to support the developer).

Is there really nothing in the pipeline to determine atmospheric composition of exoplanets? That seems like a rather important and worthy endeavor...

Query for exo-planet experts. Can we detect absorption spectra of transuranic elements in their atmosphere? To me it seems perhaps a good test for intelligent life. But I am certainly NOT in this field so I could be way off base.

Us lab scientists always poke fun at the modelers, we do the real hands-on science, theirs is just made up. Hence I would think the models here are way over-simplified and the findings are quite far from the truth, however on the other hand, the scientist modelers always accuse the lab scientists of messy data that is too hard and too slow to discover the key principles. Science is so fun! I love the friendly banter!

Anyway, the thing that tickles my mind is to imagine what kind of humanoids would develop on a tidally locked eyeball planet. Would they not need sleep? Would they find us to be extremely weird in how we shut down for 8 hours at a time?

I am faintly amused by the insistence on 100 % tidal lock rate for M star planets in the habitable zone, seeing how a) Mercury isn't, but AFAIU it should be and b) there are many mechanisms suggested why planets in general and Mercury in particular wouldn't be tidally locked. Sure, they often rely on resonances so likelihoods, but that would suggest that we somehow live in an atypical system.

Also, the comparison surface pressure of the atmosphere of a standard Earth atmosphere constrains the results away from an exploration of the actual habitable zone. (I have just browsed the paper, but it seems the author's doesn't take the problem into account.) A denser atmosphere, and a 1.5 radius terrestrial likely would have such, offsets the greenhouse warming contribution. Heck, even the Archean Earth is suggested to have had 2 bars of N2, since it was later life (cyanobacteria) that fixed half of it into rocks.

Moving on, the (atmospheric) GCM models are the ones that seems to have solved the faint young Sun problem. There are now 2 consecutive papers where it is shown how all known constraints of the faint Sun and the atmosphere from the geological record can be met while avoiding a 100 % ice covered ("Snowball") Earth in both the Archean and after the oxygenation of the atmosphere. These papers also imply that today's situation with polar caps is the normal, most frequent, climate. [ The latest, most general paper: "Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3-D GCM", Charnay et al, Jour. Geophys. Res.]

The way we avoided 100 % ice cover is by a cloud albedo feedback over cold water, especially close to the freezing line. I would have to read the paper of course, but it seems disheartening that the cloud albedo effect is hit by including ocean circulation. "It is found that cloud albedo is lower in AOGCM than in AGCM. This is because ocean heat transport reduces day–night temperature contrast in AOGCM, which consequently leads to weaker convections on the dayside and thus lower cloud albedo. The lower cloud albedo causes higher Ts in AOGCM." [Which higher Ts doesn't help. It depends on whether the clouds are low or high.]

Possible typo:

Quote:

After giving the model 1,100 years to come to equilibrium, the authors sampled a century of its climate.

The Methods's section says "All AOGCM simulations are run for about 2,200 Earth years, andthe results shown here are based on averages over the last 100 y."

This is crucial and bad, since the AGCM typically needs 40 years, and 80 to check against onset of glaciation. With 50 times runtime increase, still without land masses*, it will take some time to explore the effects in such models!

*The above paper checks for land mass contributions in the atmospheric GCM, since the Archean continents likely were much smaller. It doesn't make much of a difference there, as it did for 1D models.

Not to pick nits, but the Earth's environment has changed radically over its billions of years of history. Are there ways to tweak the model to figure out the evolution of a primordial atmosphere in a tidally-locked world without jumping right to the nitrox air that seems to bathe every planet in the Star Trek universe?

Well, they did use a specific Earth model and they wanted to compare this first run with our baseline. A bit at least, because they made at least a weak attempt to change parameters to a larger terrestrials. (I need to read the paper, but so far it seems to me they changed g, because they could easily, but not the total pressure, because they couldn't easily.)

The primordial atmosphere wouldn't be very useful in all cases, because it is thought that the hydrogen and helium would soon vanish in terrestrials up to our size at least. (In a planet of Earth size, any hydrogen of more than 30 % volume of the atmosphere would escape hydrodynamically.)

The primordial atmosphere in our case was mostly replaced with volatiles released by the differentiated, hot, planet and by impactors. That would happen with terrestrials until they get closer to 2 Earth radius, where gravity can keep their initial atmosphere (mini-Neptunes).

Habitable planets, 1-2 Earth radii*, would mostly have N2/CO2/H2O atmospheres (cf Earth & Mars), and perhaps CH4 and/or O2. Simply because CHON elements, those that also makes up the bulk of our cells, are so common.

*If such planets were heavily carbonated, more than 30 % C by mass, they would have no water but a dominantly CO2 atmosphere instead. [I have a ref, it's a new result. Somewhere...]

While this is a nice 'breakthrough', I am curious, is this going to get further study? More specifically, will the JWST, or similar Kepler-like satellites be launched to figure out the composition of these exoplanets?

"A new year is a good time to make long-term plans, and NASA has jumped into the deep end of planning. On 20 December the US space agency’s astrophysics division released a wish list of future space missions — looking three decades into the future, and even beyond." [ http://www.scientificamerican.com/artic ... e-missions ]

"The Formative Era ["notional missions ... that could designed and build in the 2020s"] goal is to characterize the surfaces and atmosphere of planets around nearby stars through direct imaging and spectroscopy. The Large UV-Optical-IR (LUVOIR) Surveyor will enable ultra-high-contrast spectroscopic studies to directly measure oxygen, water vapor, and other molecules in the atmospheres of exoEarths,..." [ http://science.nasa.gov/media/medialibr ... p_2013.pdf ]

The Near-Term Era is (from NASA) JWST and WFIRST-AFTA, both of which will be capable of characterizing stellar disks and giant planet atmospheres, and of course the current growing capabilities of ground based giant telescopes.

If NASA gets money (they are currently getting less and less), they could launch a mission in the 20-30's which would get results a few years after. (It takes IIRC 10 transits, perhaps 2-3 times more, to get good enough SNR to look for water, say. So we have to start look for M star habitables with their 30-ish days HZ orbital periods to get results anytime soon.)

Why does this article have such a generic image of a star and a planet instead of a diagram/image that actually relates to the topic itself? Ars writers are getting lazier by the day ... we should also have a voting system for the authors themselves so the users can vote on the quality of the articles written.

EDIT: That brings me to another question: how would mountains work on a tidally-locked world like this? Would their permashadow have some interesting biosphere implications? Now I'm all curious...

Depending on how long it takes to run these simulations, they'll probably just "paint" features onto the planet in a few specific patterns (one big sea-mount, east-west stripes, etc.) then let it run a few times with random combinations. Geologists might have something to say about how nonmoving sunlight would affect tectonics (we can actually detect the slight contraction of the Earth's crust during an eclipse).

I assume the main effects have nothing to do with sunlight and far more to do with gravitational differences.

Either way, it's really fascinating that climatological/geological simulations have progressed as far as they have.

Ha, I was just wondering the same thing - geologically, how would being tidally locked affect the planet? Would the lifespan of a molten core be less? Would it be "geologically inert"? If so, this presents a problem with magnetospheric shielding...